1. Field of the Invention
This invention relates generally to subsea well drilling, and in particular to a system that allows a well to be drilled from a jack-up rig and completed from a floater rig.
2. Description of the Prior Art
Subsea wells are typically drilled from either a jack-up rig, which is bottom supported, or a floater rig. When drilling from a jack-up rig, mudline suspension equipment is normally used. In a mudline suspension system, conductor pipe will be installed to a first depth in the well, usually by driving or jetting the conductor pipe. The conductor pipe, which is typically 30 inches in diameter, extends above the sea floor to the surface. The well will then be drilled to a second depth for receiving a string of outer casing, typically 20 inches in diameter.
A first hanger usually lands on a landing ring and shoulder provided in a hanger section located in the conductor pipe approximately at the sea floor. The first hanger supports the string of outer casing. Then after drilling the well deeper, a second hanger will latch into the first hanger, supporting an intermediate string of casing, typically 133/8 inches in diameter. After drilling deeper, a third hanger will support an inner string of casing, typically 95/8 inches in diameter. In some cases another string of casing will be run inside the 95/8 casing.
Each string of casing has a riser section extending from the mudline hanger to the surface vessel. The blowout preventer stack will be located at the surface on the drilling rig. The various hangers do not seal to each other at the subsea floor, rather simply latch in place.
After drilling and testing of the well is completed, it is usually temporarily abandoned. Generally in the prior art, after the mudline suspension system well is abandoned, then it will be tied back to a high pressure housing adapter. The wellhead adapter is located subsea. At least one casing hanger annulus permanent seal will be installed in the high pressure wellhead adapter. These installations are all done using a jack-up rig. Then, a high pressure riser is connected between the adapter and the rig, and a blowout preventer is located at the surface to allow running of a tubing hanger.
In a subsea wellhead system drilled by a floater, an outer wellhead housing will be supported on a guide base at the subsea floor. The conductor pipe secures to the lower end of the outer wellhead housing and does not extend to the surface. After drilling the well for 20 inch casing, a high pressure or inner wellhead housing lands in the outer wellhead housing. The 20 inch casing secures to the lower end of the high pressure wellhead housing. A riser extending to the floater at the surface will then be connected to the inner wellhead housing. A subsea blowout preventer stack locates in the riser near the sea floor.
After drilling the well for each string of casing, a subsea casing hanger lands in the inner wellhead housing. Casing hanger seals are employed to seal between the casing hangers and the high pressure wellhead housing as each string of casing is run. During drilling with subsea wellhead equipment, no casing access to the surface is necessary since the blowout preventer stack is located atop the high pressure wellhead housing.
For economic reasons that currently exist, it would be desirable to drill a well from a jack-up drilling rig and complete the well for production from a floater rig. Various techniques to accommodate this rig change have been proposed, but they are expensive and cumbersome. The conversion equipment is extensive and consists of numerous pieces. One problem is that the equipment must be designed so that the subsea blowout preventer stack required for completion by a floater will be supported by the conductor pipe. A typical mudline suspension system may not be able to accommodate the large bending moments generated by the floater through the riser, ball joint, blowout preventer stack. As the mudline suspension system lacks an outer wellhead housing, the conductor pipe is required to support these large bending moments. Adaptations to provide this support are difficult, expensive to install, and are peculiar to each well installation.
At least in one instance, a well was drilled wherein after the drilling, a subsea high pressure wellhead profile was in place to accept a subsea blowout preventer stack to be run from a floater. In that well, no outer wellhead housing was used. A special casing hanger was employed to land in the conductor pipe, the casing hanger having a larger diameter casing riser extending upward than the casing extending below. A high pressure wellhead secured to the casing riser. A high pressure riser was run above the high pressure wellhead.
There were several shortcomings in that prior technique, one of which required a different diameter of casing below the high pressure wellhead than above. It also required overshot washout sleeves, which are expensive and have marginal performance. Also the wellhead was smaller in diameter than of a type preferred.